1. Field of the Invention
The present invention relates to a method and a device for forming a setpoint torque of a drive motor, in particular in connection with an operating mode of the drive motor having closed-loop speed control.
2. Description of Related Art
Rapid load changes or gear-change operations in motor vehicles can cause jerking, which is annoying to the driver and has a detrimental effect on the driving comfort. Known methods for reducing judder vibrations are based on avoiding an excitation of the drive train due to rapid load changes. In rapid variations, the setpoint drive torque requested by the driver via the drive pedal (or by driver-assistance systems) is therefore low-pass filtered with the aid of reference-forming elements, and/or its rate of change is restricted. This causes a delay in the torque generation and reduction.
In addition, measures are taken in zero crossings of the drive torque, i.e., in the transition from overrun operation to acceleration operation. The related zero crossing of the reaction torque causes tilting of the engine transmission in the bearings. This transition should be “soft” for reasons of comfort, which is achieved by limiting the rate-of-change of the setpoint drive torque during the passage through zero. As a rule, both measures are implemented, the rate-of-change limitation being applied after the requested setpoint drive torque has been low-pass-filtered.
The interaction becomes problematic with an additional speed controller or idle controller which specifies a controller setpoint torque that is to prevent such things as, for example, chocking of a combustion engine used as drive motor. In order to ensure a response of the vehicle even to a slight actuation of the driving pedal, the controller setpoint torque is cumulatively incorporated in the requested setpoint drive torque.
Adding the controller setpoint torque to the setpoint drive torque (prior to the filtering and rate-of-change limitation) would be advantageous from the viewpoint of the reference formation, since the cumulative setpoint torque resulting at the output of the rate-of-change limitation would then have an appropriate form. However, an influencing of the controller setpoint torque by the following reference formation is not practicable from the viewpoint of the closed-loop speed control. For one, the low-pass filtering delays controller setpoint torque, which causes a delay in a compensation torque induced by the closed-loop speed control, so that, for instance, choking of the combustion engine becomes more likely. For another, the behavior of the controlled system varies considerably due to the non-linearity in the rate-of-change limitation, which requires a very robust controller and thus has a considerable adverse effect on the quality of the closed loop control.
From the viewpoint of the closed loop speed control, it is advantageous to include the controller setpoint torque in the signal flow following the reference formation. The controller setpoint torque is then added to the setpoint torque limited by the rate-of-change limitation, so that the controller is thus able to intervene directly in the cumulative setpoint torque resulting from the addition. The controller setpoint torque must then additionally be taken into account in the rate-of-change limitation, since it is the zero crossing of the cumulative setpoint torque that is to be formed and not the zero crossing of the setpoint torque limited by the rate-of-change limitation. Furthermore, there are problems in the transitions between rpm-regulated operation close to idling speed and torque-controlled operation above idling speed due to the fact that the gradient of the controller setpoint torque has an additional effect on the gradient of the cumulative setpoint torque.
Without actuation of the driving pedal by the driver, a negative setpoint drive torque results so as to enable overrun operation of the drive. At higher speeds, the controller is not active and the controller setpoint torque is zero. The combustion engine is operated using small injection quantities or is operating with deceleration fuel cutoff. If the rotational speed drops in the direction of idling speed, then the controller intervenes by a controller setpoint torque that is greater than zero and compensates the negative setpoint drive torque, and (in a frictional connection) a load torque, which is caused by running resistance (aerodynamic, rolling, climbing resistance, etc.).
When the driving pedal is actuated during the rpm-regulated operation, there is an increase in the setpoint drive torque and the (reference-formed) limited setpoint torque together with an acceleration of the vehicle. The rise of the rotational speed leads to a reduction in the controller setpoint torque. This partially compensates for the rise in the limited setpoint torque, or it may even cause undershooting in the cumulative setpoint torque. In both cases the gradient of the cumulative setpoint torque deviates from the optimally vehicle-adjusted gradient of the setpoint torque limited by the rate-of-change limitation. The acceleration no longer progresses optimally, and the achievable dynamic performance is limited. The undershooter may cause the cumulative setpoint torque to cross zero multiple times, combined with poor behavior of the drive in a load change.